This invention relates to an upshift control apparatus for controlling friction elements used in an automatic transmission to prevent engine racing, particularly, at an early stage of upshifting. The term "upshift" as used throughout this invention is intended to mean "shifts from a given gear ratio to a higher gear ratio".
Automatic transmission upshift control apparatus have been proposed, for example, in Japanese Patent Kokai No. 4-211760 and 5-263902 and Japanese Utility Model Kokai No. 1-113655.
Japanese Patent Kokai No. 4-211760 and 5-263902 disclose upshift control apparatus arranged to prevent engine racing by increasing the fluid pressure to the friction element to be released during upshifting. However, these apparatus cannot prevent engine racing resulting from an insufficient torque capacity of the apply element although they can prevent engine racing resulting from an insufficient torque capacity of the release element. The apply element may have an insufficient torque capacity when the fluid pressure decreases because of fluid pressure variations, the apply element has a friction coefficient reduced, or the torque increases because of engine torque variations. The insufficient torque capacity of the apply element will result in engine racing to provide a degraded shift feel, a degraded acceleration performance, an increase in the time required for the upshift, and a "shelf-off" shock. In Japanese Patent Kokai No. 4-211760 and 5-263902, it is proposed to suppress engine racing in the range A of FIG. 9 by increasing the releasing pressure of the release element. When the releasing pressure is increased, however, a torque drop occurs due to such interlock that the releasing and engaging pressure overlaps each other. In FIG. 9, the character X indicates "engine racing" and the character Y indicates "shelf-off".
Japanese Utility Model Kokai No. 1-113655 discloses an upshift control apparatus which includes an inertia phase start detecting means for detecting the start of a gear ratio change to a target gear ratio, and a line pressure adjusting means for outputting a command to increase the line pressure during shifting if the inertia phase is not started after a predetermined time has been elapsed. However, such a conventional upshift control apparatus cannot solve the following problems: First, it is impossible to provide a feedback control so as to suppress the detected engine racing in a real-time fashion. Secondly, it is impossible to improve the shift quality if a touble occurs in the early stage of the shift operation before the inertia phase starts since the control to increase the fluid pressure becomes effective only after the start of the intertia phase is detected. For example, when engine racing occurs in the interval between the time at which the shift operation starts and the time at which the inertia phase is detected, the inertia change .DELTA.N.sub.T and output torque increase to increase the tendency toward "shelf-off" even though the line pressure is increased a predetermined time T1 after the start of the shift operation, as shown in FIGS. 10A and 10B. In FIGS. 10A and 10B the character X indicates "engine racing" and the character Y indicates "shelf-off".